On cranking up the negative voltage on the collector plate until the current just stops, that is, to Vstop, the highest kinetic energy electrons must have had energy eVstop on leaving the cathode. Thus,

eVstop = hf - W

But still cant understand why maximum kinetic energy of the electrons. PLease help!

Well, semiconductive materials tend to have a 'work function' which is basically the energy difference between, I believe, the conduction and valence bands in the atom. This corresponds to a certain voltage needed simply to free the electron from the atom (and give it a Kinetic Energy + Potential Energy = 0); any additional energy will go into accelerating the freed electron. Due to conservation of energy, the kinetic energy of the electron can only correspond to the remaining voltage defined by (voltage in - band gap voltage). I think this is called 'maximum' because in reality electrons aren't really the classical type of objects and you have to think in terms of statistics and not concrete principles. That is to say that the electrons need not absorb all that energy available (and hence have a lower KE than the maximum available).

Well, semiconductive materials tend to have a 'work function' which is basically the energy difference between, I believe, the conduction and valence bands in the atom. This corresponds to a certain voltage needed simply to free the electron from the atom (and give it a Kinetic Energy + Potential Energy = 0); any additional energy will go into accelerating the freed electron. Due to conservation of energy, the kinetic energy of the electron can only correspond to the remaining voltage defined by (voltage in - band gap voltage). I think this is called 'maximum' because in reality electrons aren't really the classical type of objects and you have to think in terms of statistics and not concrete principles. That is to say that the electrons need not absorb all that energy available (and hence have a lower KE than the maximum available).

Please note that the standard photoelectric effect experiment is done on metals, not semiconductors, which would have added a different level of complexity. So the question between the conduction and valence band isn't valid here within this scenario.